1. Field of the Invention
The invention relates to a local endo coil for intracorporeal placement for recording magnetic resonance signals, having an outer balloon and, provided in the interior thereof, a coil conductor, which two can be deployed for the purpose of receiving signals by means of feeding a filling medium.
2. Description of the Related Art
Local coils are frequently used in imaging by magnetic resonance tomography for the purpose of locally raising the sensitivity during reception of the magnetic resonance signal. The raised sensitivity leads to a higher signal-to-noise ratio and thus to improved imaging in the surroundings of the local coil. Local coils that are inserted into body openings, for example, the rectum, are sometimes used for recording images from the body interior. With these local endo coils, the metallic structure, that is to say the actual coil loop, that picks up the signals is surrounded by an elastic balloon. The balloon and, possibly, a further inflatable plastic structure on which the coil loop is suspended and which is arranged inside the balloon, are filled from outside with air via feeders. The air-filled outer balloon produces the space required in the body interior to deploy the coil loop, for example by means of the further inflatable plastic structure. The deployed coil loop can bear directly against the inner wall of the balloon over long sections, and is then separated from the patient's tissue only by the slight wall thickness of the balloon. In certain circumstances, for example given maloperation or cable rupture or the like, elevated high-frequency currents can occur on the coil loop and/or electric supply lead thereof which are driven by the transmitting body coil of the magnetic resonance tomograph, and, as displacement currents, enter the adjacent tissue, where a high increase in local specific absorption rate (SAR) occurs. The high current density, which comes about precisely wherever the coil loop lies closest to the tissue, that is to say wherever the capacitance is greatest, leads to an impermissibly high thermal loading of the tissue, as far as that of local burns.
Sheath wave traps can be used to prevent the high sheath currents on the coil loop or supply lead thereof. However, for reasons of cost these are not used with the local endo coils, which are intended as a rule to be used once.